Spiro ortho carbonate monomers with various allyl groups can be polymerized by free radical mechanisms. Resulting polymers have lower densities than the monomers. Various spiro ortho carbonate monomers are synthesized and being used to modify currently utilized dental resins to control polymerization shrinkage. Increases in adhesion to tooth structure, increased strength and reduced abrasion under thermal loadings are under investigation for dental adhesives and composites. Toxicity screening of canidate systems is beginning. Polymerization stress are monitored by magnetostrictive transducer techniques. Expanding monomers are added as finely dispersed solids to the reaction mixture and are likely to enter into the reaction near the gel point as their reactivity is less than that of BIS-GMA monomers.